Tearing down San Onofre's two nuclear reactors will be a technically complex job completed over decades. It's likely Southern California Edison will first mothball the plant.

Southern California Edison built San Onofre's two nuclear reactors in about nine years, but tearing them down will be a technically complex, multibillion-dollar job completed over decades.

It is likely that Edison first will mothball the plant, which under federal rules could keep its imposing imprint on the Orange-San Diego County coastline for another half-century.

When the plant does come down, it will be a massive job.

Tons of highly radioactive fuel now stored in pools will have to cool before the rods can be moved to concrete pads outdoors. Giant pipes that extend more than a mile into the ocean will have to come out. Pieces of the reactors will have to be cut with special saws and torches that reach 20 feet into the vessels' cooling water.

“They do very highly engineered cuts and stack the pieces like Pringle potato chips,” said John Christian, a division president at EnergySolutions, which is decommissioning a nuclear plant in Illinois that also has two large reactors similar to San Onofre’s.

Three-foot-thick walls of steel-reinforced concrete at San Onofre will have to be fractured by mechanical shears and carefully hauled out. Wrecking balls and dynamite are seldom, if ever, used in decommissioning nuclear plants.

Debris then would be sent on special rail cars to dumps that can accommodate low-level radioactive waste. Right now, the only two such sites in the U.S. are in Utah and Texas.

An estimated 3 million pounds of spent fuel at San Onofre is so radioactive that no repository exists that can handle it, meaning it will have to remain in concrete casks on the coast for decades, if not indefinitely.

"It is a difficult job but not impossible," said Kevin Crowley, director of the nuclear and radiation studies board at the National Research Council. "The difficulty is separating the contaminated parts."

Edison officials said Friday that they would permanently shut down the two reactors after an effort to replace the plant's steam generators resulted in an outage that has lasted more than a year. The tubes inside the new generators began wearing out more quickly than expected and sprang a small leak of radioactive steam before the plant was shut down last year.

Edison officials have estimated that the ultimate cost of closing San Onofre will be $3 billion.

The company has $2.7 billion in a trust fund, money collected from its customers each time they pay for electricity. Separately, the shutdown could cost the company hundreds of millions in lost profit.

The decision to close the plant may affect nuclear utilities around the country that also must decide whether to make massive investments in old plants. A dual reactor plant in Florida was shut down this year when its steam generator replacement went awry.

"This is now four reactors that shut down this year for the same reason — nuclear economics," said Dave Lochbaum, director of nuclear safety projects at the Union of Concerned Scientists and a former nuclear plant operator. "As older plants face major cost updates, operators are going to be looking at this."

Edison has yet to formally notify the Nuclear Regulatory Commission about its plans to close the plant, let alone its long-range plan to decommission it, according to a company spokesman. At a news conference Friday, Edison executives said it would take "multi-decades" to decommission the plant, whereas industry experts say it takes about 10 years to do the actual demolition work.

The NRC allows utilities to either tear down old reactors or put them into what it calls safe storage for up to 60 years. Eight nuclear power plants around the country are in safe storage, while 16 reactors are in the process or have completed decommissioning, according to NRC records.

The decision on when to tear down a plant depends on many factors, including worker safety, cost, contamination and future land use, experts say.

By allowing the plant to sit for decades, some radioactivity will decay and workers will get less exposure during the demolition. In addition, the San Onofre site has had leaks of radioactive tritium and some contamination is believed to exist in the groundwater under the plant. Tritium decays quickly, and waiting three decades could mean the cost of cleaning it would be substantially reduced.

"There is no need to proceed quickly," former NRC Chairman Dale Klein said. "The most important issue is worker safety."

Edison may see little benefit in moving quickly. Rail spurs that would carry the contaminated debris, for example, would pass near residential neighborhoods, potentially creating another controversy for the company.

But other experts argue that it is better policy to clean up contaminated reactors as quickly as possible.

"The cost is certain now, the technology is in hand and disposal routes are known," said Christian, the official at EnergySolutions.